<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    Array.prototype.myReduce = function(fun, initiavalue) {
  // 判断是否传入初始值
  let accumulator = initiavalue !== undefined ? initiavalue : this[0];
  // 循环遍历数组，更新累加器的值
  for (let i = initiavalue !== undefined ? 0 : 1; i < this.length; i++) {
    accumulator = fun(accumulator, this[i], i, this);
  }
  // 返回累加器的最终值
  return accumulator;
}
let arr=[1,2,3,5,8]
let total=arr.myReduce((sum,item)=>{
  return sum +=item
},0)
console.log(total);
  </script>
</body>
</html>